1. Field of the Invention
The present invention generally relates to a distortion removing method and an image processing apparatus used in, for instance, a so-called "video phone communication system". More specifically, the present invention is directed to such a distortion removing method and an image processing apparatus capable of eliminating coding/decoding distortions caused while input image data are coded/decoded.
2. Description of the Prior Art
Recently, so-called "video phone communication systems" and also "video conference communication systems" have been developed in which moving-picture signals are transmitted via the public telephone signal lines between the long distance terminals. As is known in this technical field, since the signal transmission capacity of the public telephone line is small, or limited to a very small value, if a moving-picture signal is transmitted via the public telephone line without performing any signal coding/decoding processing, a very heavy load is placed on this public telephone line. To avoid such a problem, an input image (moving picture) signal is once coded at a transmitter end, and then the coded image data is decoded into a desirable image (moving picture) signal at a receiver end. In general, a coding/decoding distortion appears in the reproduced (decoded) images.
To remove the coding/decoding distortions from the reproduced images displayed on monitors at the transmitter and receiver ends, various techniques have been proposed, for instance, transactions of the Institute of Electronics, Information and Communication Engineerings, Vol. 6, D-3 and D-63, issued on Aug. 15, 1989 "Non-recursive type 5.times.5 adaptive smoothing filter; Improvements in image quality for high-efficiency coding operation by post filtering" by KATO et al.
The contents of this conventional distortion removing method will now be summarized. As shown in FIG. 1, a central pixel is image-processed with reference to pixels located within a limited region defined by 5 pixels.times.5 pixels. In accordance with this conventional smoothing filter, only such pixels that own the values within a range of the value of the central pixel .+-..epsilon. (i.e., a preselected constant which has been determined based on experiences). are employed so as to remove the above-described coding/decoding distortion. Then, a weighed arithmetic mean value calculated from these pixels' values is used as a new value of the processed pixel. It should be noted that when a total number of the pixels among 25 pixels, the values of which are within the range of ".+-..epsilon.", are equal to, or lower than "n" ("n" being an arbitrary integer), no change or substitution of the above-described value of the pixel to be processed is carried out. This is because a very small change occurring in the values of the referred pixels does not cause any change in the new pixel.
This conventional distortion removing method has such drawbacks that the total calculation amount becomes great, and thus a large number of memory devices required for performing these calculations are similarly needed, resulting in a large-scaled image processing apparatus.
To overcome the above-explained drawback, it may be readily conceived to reduce a total number of pixels to be referred for the average calculation, as represented in FIG. 2. However, if the total quantity of the pixel to be referred would be reduced, as compared with that of FIG. 1, distortion cannot be removed with satisfaction.
On the other hand, another distortion removing method has been proposed in transactions of the Institute of Electronics, Information and Communication Engineerings Vol. J66-A No. 10 "Recursive .epsilon.-Nonlinear Digital Filter" K. ARAKAWA et al. October 1983, pages 947 to 854, which discloses the method for removing the coding/encoding distortion at higher efficiencies by employing the recursive filter. In accordance with this second conventional distortion removing method, the coding/decoding distortion can be removed to a satisfactory extent. Nevertheless, when the image pixel series whose pixel values are varied at a constant inclination as represented in FIG. 3A, the deviation in the values of the processed image pixels from those of the unprocessed image pixels is emphasized in accordance with progress in the processing operation, as indicated in FIG. 3B. Then, if the difference in the pixel values between the image pixels to be processed and those to be referred becomes higher than a predetermined value ".epsilon.", the filtering process of this recursive filter is interrupted. In other words, the weighed mean value calculated from the values of the referred image pixels is no longer used as the new pixel value of the image pixel to be processed, and therefore the inputted pixel value is directly used as this new pixel value. On the other hand, as explained above, the filtering process of this second conventional method is turned ON/OFF based upon comparison results between a predetermined value ".epsilon." and the actual pixel values of the inputted image pixel series, there are newly caused distortions which do not appear before the above-described coding process is carried out.
Also, generally speaking, the wider the size of the quantizing step becomes, the larger the coding distortion becomes. Accordingly, for example, Applicants have proposed "A Noise Reduction Filter for MC-CDT Coding", No. D-306, Mar. 5, 1990, Institute of Electronic, Information and Communication Engineerings. That is, when a judgement whether or not the coding/decoding distortions are deleted every pixel, is executed based on comparison results between a predetermined vale ".epsilon." and a difference in the values of the processed pixel and referred pixel, the differences in these values of the processed pixels and referred pixels become great, as the sizes of the quantizing steps become wide. Accordingly, the total number of such judgements are similarly increased. In FIG. 4, there is shown a circuit block diagram of the receiption side of this noise reduction filter. In this receiption circuit of FIG. 4, the coded image data sent from the transmission side of this noise reduction filter (not shown) is decoded in the decoding unit 500 thereby to produce the decoded image data. Thereafter, the decoded image data is further processed in the noise reduction filter 510 so as to obtain distortion-free image data. Note that the sizes of the quantizing steps are detected from the coded image data by the decoding unit 500, and then the quantizing step sizes are converted in the conversion table 520 into the parameters which will be supplied to this noise reduction filter 510. The parameters control strengths of the filtering operation. In other words, the sizes of the quantizing steps are reflected as the strengths of the filtering operations.
However, the last-mentioned conventional distortion removing method has such a problem that when the sizes of the quantizing steps are intensively vibrated as time passes, the parameters to be supplied to the distortion removing filter 510 are also intensively vibrated in response to such rapidly changes in the quantizing step sizes. As a result, if the parameters to be supplied to the filter 510 would be varied every pixel as being executed in other conventional method, another problem may be produced in that peculiar differences would appear in the resultant image qualities caused by the filtering operations of the filter 510 in case that the sizes of the quantizing steps are intensively vibrated (for instance, several size-changes per 1 second).
As previously described in detail, there are various problems in these conventional image processing apparatuses. That is, firstly, since the filter for removing the coding/decoding distortion are controlled by comparing a predetermined value ".epsilon." with the difference between the values of the processed pixels and the values of the referred pixels, another coding/decoding distortion is newly produced, which is different from the above-described coding/decoding distortion, when the difference between the values of the processed pixels and the values of the referred pixels is equal to a predetermined value ".epsilon.".
Secondly, when the sizes of the quantizing steps are intensively vibrated in view of temporal aspects, the strengths of the filtering operations by the distortion removing filter are intensively vibrated, so that peculiar differences appear in the image qualities of the reproduced images when the filtering operation is strongly effective, or weakly effective.